A parallel optical communications module is a module having multiple transmit (TX) channels, multiple receive (RX) channels, or both. A parallel optical transceiver module is an optical communications module that has multiple TX channels and multiple RX channels in TX and RX portions, respectively, of the transceiver module. The TX portion comprises components for transmitting data in the form of modulated optical signals over multiple optical waveguides, which are typically optical fibers. The TX portion includes a laser driver circuit and a plurality of laser diodes. The laser driver circuit outputs electrical signals to the laser diodes to modulate them. When the laser diodes are modulated, they output optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system of the transceiver module focuses the optical signals produced by the laser diodes into the ends of respective transmit optical fibers held within a connector that mates with the transceiver module.
Typically, the TX portion also includes a plurality of monitor photodiodes that monitor the output power levels of the respective laser diodes and produce respective electrical feedback signals that are fed back to the transceiver controller. The transceiver controller processes the feedback signal to obtain respective average output power levels for the respective laser diodes. The transceiver controller outputs control signals to the laser driver circuit that cause it to adjust the modulation and/or bias current signals output to the respective laser diodes such that the average output power levels of the laser diodes are maintained at relatively constant levels.
The RX portion includes a plurality of receive photodiodes that receive incoming optical signals output from the ends of respective receive optical fibers held in the connector. The optics system of the transceiver module focuses the light that is output from the ends of the receive optical fibers onto the respective receive photodiodes. The receive photodiodes convert the incoming optical signals into electrical analog signals. An electrical detection circuit, such as a transimpedance amplifier (TIA), receives the electrical signals produced by the receive photodiodes and outputs corresponding amplified electrical signals, which are processed in the RX portion to recover the data.
There is an ever-increasing demand in the optical communications industry for parallel optical communications systems that are capable of simultaneously transmitting and receiving ever-increasing amounts of data. To accomplish this, it is known to combine multiple parallel optical transceiver modules of the type described above to produce a parallel optical communications system that has a higher bandwidth than the individual parallel optical transceiver modules. A variety of parallel optical transceiver modules are used in such systems for this purpose. For example, one known parallel optical transceiver module of the type described above includes a multi-fiber connector module known in the industry as the MTP® connector module. The MTP connector module plugs into a receptacle that is secured to a front panel of a rack of the optical communications system. The MTP connector module receives a duplex fiber ribbon cable having a total of 4, 8, 12, 24, or 48 optical fibers. Typically, half of the fibers of the ribbon cable are transmit fibers and the other half are receive fibers, although all of the fibers may be either transmit or receive fibers in cases where the module is being used as either a transmitter or a receiver, but not both. When the MTP connector module is plugged into the receptacle, electrical contacts of the connector module are electrically connected with electrical contacts of a printed circuit board (PCB) of the transceiver module. The laser diodes and the photodiodes are integrated circuits (ICs) that are mounted on the PCB. A laser driver IC and a transceiver controller IC are typically also mounted on the PCB, although the transceiver controller IC is sometimes mounted on a separate IC, known as the motherboard IC of the optical communications system.
It is known that multiple transceiver modules of the type that use the MTP connector module can be arranged in an array to provide an optical communications system that has an overall bandwidth that is generally equal to the sum of the bandwidths of the individual transceiver modules. One of the problems associated with such an array arises from the fact that the MTP connector modules are “edge-mounted,” i.e., plugged into receptacles formed in the front panel of the rack of the optical communications system. Because the modules are edge-mounted, there must be sufficient space on the front panel to accommodate the receptacles and the respective MTP connector modules. Because space on the front panel is limited, the ability to increase bandwidth by increasing the size of the array is also limited.
An alternative to edge-mounting parallel optical transceiver modules is to mid-plane mount parallel optical transceiver modules. A mid-plane mounting configuration is one in which the modules are mounted in the plane of the motherboard PCB. One known parallel optical transceiver module that is mid-plane mounted is known in the industry as the Snap 12 transceiver module. The Snap 12 transceiver module comprises a 12-channel TX module and a 12-channel RX module. Each module has an array of 100 input/output (I/O) pins that plugs into a 100-pin ball grid array (BGA), known as a Meg-array. The Meg-array is, in turn, secured to the host PCB motherboard. The Snap 12 system is typically mounted in a box, which is connected to multiple electrical cables, which, in turn, are connected to multiple router ICs. In order to increase the total bandwidth of an optical communications system that uses multiple mid-plane mounted Snap 12 transceiver modules, multiple boxes may be used. The boxes are typically mounted in racks. The racks needed to accommodate a large number of boxes and the cables needed to interconnect the boxes to the router ICs consume a large amount of space and generate a large amount of heat. The space consumption and heat generation problems must be dealt with in order to make the system operate properly. Consequently, a system that is constructed of multiple boxes in order to achieve an increased bandwidth is generally very expensive.
Other mid-plane mounting solutions exist or have been proposed for mounting multiple parallel optical transceiver modules on a motherboard PCB. One of the problems associated with the existing or proposed mid-plane mounting solutions is that there are limitations on the mounting density of the modules on the motherboard PCB. Each module consumes spatial area, or footprint, on the surface of the motherboard PCB and the modules must be spaced apart from adjacent modules on the motherboard PCB by some minimum spacing, or pitch. Because there is a finite spatial surface area on the motherboard PCB for mounting the modules, the mounting density of the modules is limited, which limits the overall bandwidth of the system.
Accordingly, a need exists for an optical communications system having a mounting configuration in which parallel optical transceiver modules are capable of being mounted with very high density on the motherboard PCB. Increasing the mounting density of the modules increases the amount of data that can be simultaneously transmitted from and received in the optical communications system.